In the packaging industry, particularly in food processing, it is common to package perishables in metal containers. The contents are typically inserted through an open end of the container which is subsequently sealed. In order to prevent contamination and subsequent spoiling of perishables packed in such containers, it is essential that the seam used to seal the container is airtight.
During packaging a small pecentage of containers develop leaks or othewise are not sealed with an airtight seal. It is particularly important in the food packaging industry to detect such defectively sealed containers before the containers are shipped to retail outlets and subsequently purchased by a consumer. It is also important to detect unsealed containers early in the processing or packaging cycle to minimize the cost associated with further processing and handling such containers.
Product quality control considerations often require testing of every container after it has been sealed. Because of this, it is not possible to destructively test the containers and a reliable non-destructive testing system must be used.
Container leak testing apparatus used to test each container should be capable of fast operation in order to handle the high throughput of many high speed packaging and container sealing processes. It is further desirable to have a container testing system which can be integrated into such high speed processing lines and used to provide reliable proof testing of the containers for leaks.
There have been numerous prior art attempts to develop reliable high speed, non-destructive container seal testing equipment. One such prior art attempt is shown in U.S. Pat. No. 1,825,744 to Landrum. The Landrum patent discloses a method for detecting leaks in sealed containers by forming a seal over the peripheral edge of the closure between an end wall and a side wall of the container. The area between the seal and the end wall of the container is then evacuated. If the container leaks, the applied vacuum causes the closed end to bulge or pop outwardly producing an audible sound. The Landrum method and apparatus are only usable with containers sealed with an internal vacuum. The Landrum system requires an operator to hear or feel the pop of a defective container when a vacuum is applied. In most packaging plants the environment is extemely noisy and any such sound cannot be reliably heard by an operator. In addition, many food processing system handle 700 cans per minute. With such a rate of processing it is impossible for a human operator to accurately detect defective containers.
Another prior art method and apparatus are disclosed in U.S. Pat. No. 2,093,429 to Foss. The Foss patent discloses a machine for testing cans which are sealed with an internal vacuum. The Foss machine evacuates the atmospheric pressure from the outside face of one end of the can. If there has been a loss of vacuum in the interior of a can, the end will flex outwardly, coming into contact with two electrical contacts which provide an electrical indication that the can has lost its internal vacuum. The Foss apparatus is usable only with cans which are sealed with an internal vacuum.
A third prior art testing apparatus is disclosed in U.S. Pat. No. 2,339,639 to Henszey. The Henszey patent discloses an apparatus for testing a specific type of can having a small opening in the center of one end of the can. The can is filled through the small opening and the opening is sealed by a drop of hot molten metal. The Henszey apparatus forms an annular seal around the small sealed opening and applies air pressure to the sealed opening. If there is a leak through the seal, the can will bulge and the apparatus will detect the bulging.